1. Field of the Invention
The present invention relates to a head supporting device for a universal parallel ruler of the truck type.
2. Description of the Prior Art
In universal parallel rulers, a cursor is movably mounted on a longitudinal rail and is coupled to an operating head, having a scale, in such a manner that they can move toward and away from the drawing board surface, and the underside of the scale can be moved toward and away from the drawing board surface by moving the operating head toward and away from the drawing board surface, respectively. It has already been well known to cancel a weight of the operating head itself by normally spring-biasing the operating head to be lifted so that the head and the scale may be smoothly operated. With such a device of prior art, however, the spring force normally biasing the head to be lifted is set too high. The underside of the operating head and, therefore, the underside of the scale are disadvantageously prevented from being kept in close contact with the drawing board surface. This problem cannot be overcome even when the spring force normally biasing the head to be lifted is set to be low. On the contrary, the underside of the operating head and the scale is kept in close contact with the drawing board surface, since the load of the operating head and the scale in a direction toward and away from the drawing board surface largely depends upon the angle of the drawing board surface relative to the horizontal. For example, when the spring force is set with respect to a drawing board surface, which is horizontal, the load of the operating head, in a direction toward and away from the drawing board surface, gradually decreases when lifting the head as the angle of the drawing board surface increases. On the other hand, when the spring force is set, with respect to a drawing board surface which is largely inclined, the head is too heavily loaded to be smoothly operated as the drawing board surface approaches its horizontal position. To overcome this disadvantage, an improvement has been proposed. There is provided a means for changing the spring force normally biasing the head to be lifted in opposite directions with respect to an intermediate position between the position at which the underside of the operating head bears against the drawing board surface and the position at which the head has been lifted to the possible uppermost level, so that the head may be lifted when the head is located higher than the intermediate boundary position and the head may be brought by its own weight in close contact with the drawing board surface when the head is located lower than said intermediate boundary position. Even with this improvement, the force acting to hold the head at the lifted position higher than the intermediate boundary position depends on the spring force and, in consequence, an attempt to increase the head holding force would often result in the spring force, which normally biases the head to be lifted, in the vicinity of the drawing board surface, exceeding the head's own weight. Furthermore, the situation wherein the underside of the head is brought by its own weight in close contact with the drawing board surface, against the spring force, involves the disadvantage that occurs with the previously mentioned device of prior art when the spring force normally biasing the head to be lifted is set to be low.